Presentation board digitizer systems

ABSTRACT

A presentation board digitizer system for large boards preferably employs at least three spaced-apart ultrasound receivers assemblies. A current position of an ultrasound transmitter is assigned as a weighted centroid of time-of-flight position measurements based on at least two pairs of receiver assemblies. The weighting used varies as a function of the position of the transmitter across the board. A preferred structure of an ultrasound receiver assembly for use in the system employs a pair of ultrasound receivers arranged side-by-side in a line perpendicular to the surface of the presentation board. The receivers are connected so as to generate a total output signal corresponding to the instantaneous sum of the ultrasound signals received at each, such that the receiver assembly is most sensitive to ultrasound signals incident from a plane adjacent to the presentation board. Also described are a transmitter device for use with a conventional pen in which the ultrasound transmitter is a cylindrical element lying coaxial with the pen and adjacent to its tip, and a jointed eraser structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/850,875, filed May 7, 2001 now abandoned, which is a continuation ofU.S. application Ser. No. 09/030,825, filed Feb. 26, 1998, now U.S. Pat.No. 6,300,580, and a continuation-in-part of U.S. application Ser. No.09/916,558, filed Jul. 26, 2001, now U.S. Pat. No. 6,424,340, which is acontinuation of U.S. application Ser. No. 08/811,947, filed Mar. 5,1997, now U.S. Pat. No. 6,292,177.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to digitizers and, in particular, itconcerns devices for use with conventional presentation boards and pensfor digitizing lines drawn manually.

It is known to use various techniques for determining the position of awriting implement or stylus on a flat surface. Glenn et al. U.S. Pat.No. 4,564,928, Suzuki et al. U.S. Pat. No. 4,886,943, Kobayashi et al.U.S. Pat. Nos. 4,910,363 and 5,073,685, and Yoshimura et al. U.S. Pat.No. 5,097,102, all disclose systems in which a vibrating elementassociated with a pen transmits vibrations through the material of aboard. The vibrations are detected by transducers attached to the boardand the position of the pen is calculated from the transmission time ofthe vibrations through the board. These systems inherently functionexclusively when the pen is in contact with the board such thatvibrations are transferred to the board. As a result, no specialmechanism is required to distinguish writing from non-writing penmovements.

These systems are generally inaccurate due to non-uniform transmissiontimes through the board. In fact, they typically require highlyspecialized board structures which renders them expensive andinconvenient.

An alternative approach is the use of air-borne ultrasound signals.Examples of such systems are described in Mallicoat U.S. Pat. No.4,777,329, Stefik et al. U.S. Pat. No. 4,814,552, Hansen U.S. Pat. No.4,506,354, and De Bruyne U.S. Pat. No. 4,758,691. These systems employvarious combinations of ultrasound transmitters and receivers arrangedat two points fixed relative to a board and on a movable writingimplement. The position of the movable implement is then derived bytriangulation. The systems typically require an additional hard-wired orelectromagnetic link between the movable implement and a base unit toprovide timing information for time-of-flight ultrasound calculations.An additional switch is also required to identify when the movableelement is in contact with the board.

These previously known systems are typically limited to relatively smallboards. This is because of signal to noise ratio (SNR) limitations. Thevolume of ultrasound used cannot be very high without causing bothersomeaccompanying whistling noises. Additionally, in a wireless system, powerconsiderations severely limit the transmitted volume. To generatereliable position information, the transmitter-to-receiver distance musttherefore be kept small. Attempts to use different sets of receivers fordifferent regions of a large board generally result in discontinuitieswhen the movable element travels from one region to another.

Another shortcoming of these previously systems is their inability toreproduce rapid interrupted pen strokes, such as performed when drawinga dashed line. Typically, the transmitter or receiver element in the penturns OFF when the pen is inactive and is re-activated each time the pencomes in contact with the board. The system then takes a fraction of asecond to resynchronize before it responds correctly. In the case ofshort strokes, the length of the operative stroke may be comparable withthe response time of the system, thereby giving very poor results.

An additional problem of the previously known airborne ultrasounddigitizer systems is that the ultrasound transmitter or receiver elementis mounted asymmetrically to the side of the drawing implement. As aresult, the measured position is offset from the true drawing positionin a direction which changes with rotation of the drawing implement.This may result in discontinuities and illegible writing in thedigitized image when the drawing implement position is changed betweenstrokes.

Finally, conventional presentation board digitizer systems are typicallylimited to use with specially produced writing implements. This rendersthem expensive since pens have a very limited lifetime. Even where theink cartridge is separately replaceable, the components used must be ofa very specific design to be compatible.

There is therefore a need for a reliable, low-cost, digitizer systemwhich may be used with conventional presentation boards of all sizes fordetermining accurately the position of a drawing implement on the board.It would also be advantageous to have a transmitter device for use withpresentation board which can be used with a wide range of conventionalwriting implements.

SUMMARY OF THE INVENTION

The present invention is of presentation board digitizer systems for usewith presentation boards of all sizes which allow accurate reproductionof short pen strokes and which may be used with conventional writingimplements.

According to the teachings of the present invention there is provided,an ultrasound receiver assembly for use in a presentation boarddigitizer system, the receiver assembly comprising: (a) a firstultrasound receiver located adjacent to the surface of the presentationboard; and (b) a second ultrasound receiver displaced from the firstultrasound receiver in a direction substantially perpendicular to thesurface of the presentation board, the first and second ultrasoundreceivers being connected so as to generate a total output signalcorresponding to the instantaneous sum of the ultrasound signalsreceived at each of the first and second ultrasound receivers such thatthe receiver assembly is most sensitive to ultrasound signals incidentfrom a plane substantially adjacent to the presentation board.

According to a further feature of the present invention, the first andsecond receivers are connected in series.

In the context of an ultrasound-based digitizing system for identifyingthe position of an ultrasound transmitter associated with an elementmovable relative to a surface, the system having at least three spacedapart ultrasound receivers associated with the surface, there is alsoprovided according to the teachings of the present invention, a methodof analyzing outputs from the ultrasound receivers comprising the stepof identifying as a current position a weighted centroid of at least afirst calculated position derived from the outputs of a first pair ofthe receivers and a second calculated position derived from the outputsof a second pair of the receivers, wherein the weighting varies as acontinuous function of approximate position relative to the ultrasoundreceivers.

According to a further feature of the present invention, the ultrasoundreceivers are substantially collinear, and the weighting varies linearlywith distance in the direction of alignment of the ultrasound receiversover at least a given switch-over zone.

According to a further feature of the present invention, the weightedcentroid approximates to the first calculated value when the movableelement is within a first given region of the surface.

There is also provided according to the teachings of the presentinvention, a presentation board digitizer system for digitizingoperative strokes of a drawing implement carrying an ultrasoundtransmitter against the board, the system comprising: (a) at least twoultrasound receivers mounted relative to the board for receivingair-borne ultrasound signals; (b) a transducer associated with the boardso as to detect vibrations from the transmitter conducted through theboard; and (c) a processor responsive to outputs from the at least twoultrasound receivers to calculate a current position of the transmitter,the processor being additionally responsive to an output from thetransducer to identify contact between the drawing implement and theboard, thereby identifying operative strokes of the drawing implement.

There is also provided according to the teachings of the presentinvention, a transmitter device for use with a system for digitizingoperative strokes of a hand-held drawing implement, the drawingimplement having a body and an operative tip, the transmitter devicecomprising: (a) a housing having a substantially cylindrical openingterminating at a first end in an annular wedge surface with a centralbore, the housing receiving a portion of the body of the drawingimplement with its operative tip extending from the central bore; (b) aretainer attachable to a second end of the opening to retain the drawingimplement within the housing, the retainer having a spring element forbiasing the drawing implement towards the annular wedge surface; and (c)a transmitter mounted relative to the housing proximal to the centralbore.

According to a further feature of the present invention, the housingfurther includes: (a) a microswitch actuated by changes in pressureexerted on the annular wedge surface so as to be responsive to a forceexerted on the operative tip of the drawing implement towards thehousing; and (b) electronic circuitry responsive to the microswitch toaffect operation of the transmitter at least when the microswitchindicates a force exerted on the operative tip of the drawing implementtowards the housing.

According to a further feature of the present invention, the electroniccircuitry operates the transmitter for a given time interval after themicroswitch ceases to indicate a force exerted on the outer housingtowards the operative tip of the drawing implement.

According to a further feature of the present invention, the given timeinterval is at least about half a second.

According to a further feature of the present invention, the transmittertransmits continuously, and the electronic circuitry is responsive tothe microswitch to change a signal transmitted by the transmitter whilethe microswitch indicates a force exerted on the operative tip of thedrawing implement towards the housing.

According to a further feature of the present invention, the transmitteris an ultrasound transducer.

According to a further feature of the present invention, there are alsoprovided elements of an electromagnetic communications link, theelements being associated with the electronic circuitry.

There is also provided according to the teachings of the presentinvention, an ultrasound transmitter device for use with a system fordigitizing the position of a hand-held drawing implement, the drawingimplement having a central axis and an operative tip, the transmitterdevice comprising a substantially cylindrical piezoelectric transmitterelement positioned coaxially with the drawing implement so as tocircumscribe a part of the drawing implement proximal to the operativetip.

There is also provided according to the teachings of the presentinvention, an eraser device for use with a presentation board digitizersystem, the eraser device comprising: (a) a handle; (b) an eraserelement having a substantially flat eraser surface; and (c) a pivotjoint connecting between the handle and the eraser element, the pivotjoint having two degrees of rotational freedom such that, in use, theeraser element assumes an orientation with the eraser surface parallelto the presentation board surface substantially independent of theorientation at which the handle is held.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic front view of a presentation board provided with adigitizer system, constructed and operative according to the teachingsof the present invention, showing a switch-over zone between regionswith different groups of ultrasound receivers;

FIG. 2 is a plot illustrating the variation of relative weighting ofposition indications from two sets of ultrasound receivers in FIG. 1 asa function of position across the presentation board;

FIG. 3 is a side view of a twin ultrasound receiver assembly for use ina presentation board digitizer system constructed and operativeaccording to the teachings of the present invention;

FIG. 4 is a schematic representation of the reception characteristic ofthe twin ultrasound receiver assembly of FIG. 3;

FIG. 5 is a side cross-sectional view of a transmitter device,constructed and operative according to the teachings of the presentinvention, used with a conventional drawing implement in a digitizersystem;

FIG. 6A is an exploded perspective view of a microswitch structure,constructed and operative according to the teachings of the presentinvention, for use in the transmitter device of FIG. 5;

FIG. 6B is a perspective view of the microswitch structure of FIG. 6Aassembled;

FIG. 6C is a top view of the microswitch structure of FIG. 6A showing aretaining spring arrangement;

FIG. 7 is a schematic perspective view of a preferred structure forattachment of a retaining member to a housing for use in the transmitterdevice of FIG. 5;

FIG. 8A is a plot of the output of a contact switch activated byoperational contact between a drawing implement and a presentation boardas a function of time;

FIG. 8B illustrates the recorded drawing implement operation timeprofile produced by prior art systems corresponding to the contactprofile of FIG. 5A;

FIG. 8C illustrates the corresponding recorded drawing implementoperation time profile produced according to a first embodiment of apresentation board digitizer system, constructed and operative accordingto the teachings of the present invention;

FIG. 9 is a side cross-sectional view of an eraser transmitter unit,constructed and operative according to the teachings of the presentinvention, for use with a digitizer system;

FIG. 10 illustrates the signals received by air-borne ultrasoundreceivers and a board-mounted transducer, respectively, according to asecond embodiment of a presentation board digitizer system, constructedand operative according to the teachings of the present invention;

FIG. 11 shows a second embodiment of the transmitting device;

FIGS. 12A and 12B show two different forms of the upper biasing element;and

FIG. 13 shows a second embodiment of the eraser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of presentation board digitizer systems for usewith presentation boards of all sizes which allow accurate reproductionof short pen strokes and which may be used with replaceable conventionalpen elements.

The principles and operation of digitizer systems according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawings, FIG. 1 shows a presentation boarddigitizer system, generally designated 10, constructed and operativeaccording to the teachings of the present invention, showing aswitch-over zone between regions with different groups of ultrasoundreceivers.

Generally speaking, system 10 features a presentation board 12, whichmay be of any conventional type, provided with a plurality of ultrasoundreceiver assemblies 14, 16 and 18. Ultrasound receiver assemblies 14, 16and 18 are preferably mounted in a strip 20 adapted for convenientattachment to presentation boards of differing sizes and thickness. Thisattachment may be achieved through clamps or clips of any type. Strip 20also features an infrared (IR) receiver 22. A cover (not shown) ispreferably provided for shielding ultrasound receiver assemblies 14, 16and 18 and IR receiver 22 from sound and radiation originating away fromboard 12. System 10 operates with a movable element having bothultrasound and infrared transmitters, an example of which will bedescribed in detail below. The present position of the movable elementis derived from the time-of-flight (TOF) of ultrasound signals from themovable element to the receiver assemblies by triangulation. The IRsignal provides synchronization information, as well as carryingadditional information such as, for example, the color of a pen beingused.

In principle, two ultrasound receivers are sufficient to uniquelydetermine the position of a movable element in contact with board 12.However, to provide reliable ultrasound reception over the entire areaof a large board, system 10 employs more than one set of receivers.Thus, in the system as illustrated, a first set of receivers is definedas the pair of ultrasound receiver assemblies 14 and 16, and a secondset of receivers is defined as the pair of ultrasound receiverassemblies 16 and 18. Clearly, the first set of ultrasound receivers sodefined is well positioned for receiving an ultrasound signal from thetransmitter when the movable element is in a first region denoted A, andthe second set of ultrasound receivers is well positioned for receivingthe signal when the movable element is in a second region denoted C.Thus, optimal precision and reliability is achieved by deriving theposition of the movable element from the outputs of ultrasound receiverassemblies 14 and 16 when the movable element is in region A, and fromthe outputs of ultrasound receiver assemblies 16 and 18 when the movableelement is in region C.

To avoid possible discontinuities in the tracking of the position of themovable element as it traverses board 12, preferred embodiments of thepresent invention provide a switch-over zone, denoted B, between regionsA and C. Within switch-over zone B, the current position of the movableelement is derived based on a weighted function of the positionscalculated from the outputs of the first and second sets of receivers.Preferably, the weighted function varies smoothly with position acrossswitch-over zone B, such that it approaches the value calculated fromthe first set of receivers when the movable element borders first regionA and approaches the value calculated from the second set of receiverswhen the movable element borders second region C.

FIG. 2 shows a typical variation of the weighting function with distanceacross board 12. Here, plot 24 corresponds to the weighting factorapplied to the first group of ultrasound receivers, and plot 26corresponds to the weighting factor applied to the second group ofultrasound receivers. In this example, the variation within switch-overzone B is shown as linear. However, it should be appreciated that othermore complex functions may be used as desired. Within region A, plot 24is preferably constant at 1 and plot 26 is zero, whereas within regionC, these values are reversed. Calculation of the current position of themovable element according to the system described requires calculationof weighting factors which are themselves a function of position. Thisapparent circularity of calculation may be circumvented in a number ofways. Most simply, since the position is measured repeatedly at shortintervals, it is reasonable to assume that the new current position is arelatively small distance from the previously measured position. It istherefore reasonable to employ the last measured position forcalculating the weighting factors for the subsequent measurement.Alternatively, or for the purposes of making an initial measurement, anapproximate measurement may be made with some arbitrary weighting factorsuch as, for example, 0.5 for each set.

Although the concept of the switch-over zone has been illustrated in asimple implementation with only two sets of receivers, the concept canclearly be extended to more complex arrangements of multiple sets ofreceivers, both co-linearly and on opposite sides of a board. In thelatter case, the weighting factor becomes a function of position in twodimensions, as will be clear to one ordinarily skilled in the art.

In addition to the switch-over zone algorithm, it is preferable that theultrasound receivers are located sufficiently close to provide somedegree of redundancy of measurement. This redundancy can then beemployed (typically independent of the switch-over zone considerations)to provide a self test for accuracy and to identify any erroneousmeasurements which may occur temporarily.

Specifically, if receivers 14, 16 and 18 are collinear with equalspacing A, and the distance from each receiver as measured by TOFcalculations is s₁, s₂ and s₃, respectively, simple trigonometrydictates that:

s ₁ ²−2s ₂ ² +s ₃ ²=2A ²(constant)

By calculating this sum whenever three simultaneous TOF measurements areavailable, the system can continuously test that it is functioningwithin an acceptable margin of accuracy. If a significant error isfound, further statistically-based self-analysis algorithms may beimplemented to identify which receiver produced the erroneous readingand to temporarily exclude that receiver from position calculations.

Turning now to FIGS. 3 and 4, a preferred design of ultrasound receiverassembly, generally designated 30, constructed and operative accordingto the teachings of the present invention, for use with presentationboard digitizer systems will now be described. Assembly 30 may be usedto advantage with a wide range of digitizer systems, including but notlimited to system 10 described above.

Generally speaking, ultrasound receiver assembly 30 includes a firstultrasound receiver 32 located adjacent to the surface 34 of thepresentation board, and a second ultrasound receiver 36 displaced fromfirst ultrasound receiver 32 in a direction substantially perpendicularto surface 34.

First and second ultrasound receivers 32 and 36 are connected so as togenerate a total output signal corresponding to the instantaneous sum ofthe amplitudes of ultrasound signals which they receive. Typically, forsimple transducers, this is achieved by connecting them in series suchthat their output voltages are additive.

FIG. 4 shows a plot in polar coordinates of the variation of sensitivityof assembly 30 with angle of incidence in a plane perpendicular to thesurface 34. The phase differences between ultrasonic vibrations reachingthe two receivers, when added, result in pronounced variation of thesensitivity of assembly 30 with angle of incidence, as shown.Specifically, the maximum sensitivity of assembly 30 occurs in a planecentral to the main lobe of FIG. 4 corresponding to a plane of symmetrybetween receivers 32 and 36. Signals arriving at the two receivers whichare incident from this plane necessarily have zero path and phasedifference, thereby producing a maximum amplitude output signal.Reception from the n=1 side lobes is preferably minimized by use of acover element (not shown) which shields assembly 30 from sound incidentat large angles from surface 34.

By arranging assembly 30 as described, the plane of maximum sensitivityis oriented substantially parallel and adjacent to surface 34. This isideal for receiving signals incident from near the presentation board(S₀). Conversely, assembly 30 exhibits greatly reduced sensitivity tosignals (S₁) incident from further away from the presentation board.These directional properties greatly help to isolate the ultrasoundsignals of importance to the digitizer system, increasing thesignal-to-noise ratio. This allows the use of lower transmitterintensities and/or larger boards, and solves problems caused by a widerange of common noise sources. The sensitivity profile of assembly 30parallel to surface 34 remains substantially omnidirectional similar tothe profile of an individual receiver.

Turning now to FIGS. 5-7, a preferred embodiment of a transmitterdevice, generally designated 40, constructed and operative according tothe teachings of the present invention, for use with a drawing implement42 in a digitizer system will be described. Transmitter device 40 may beused to advantage with a wide range of ultrasound based digitizersystems including, but not limited to, the presentation board digitizersystems described above.

Generally speaking, transmitter device 40 includes a housing 44 having asubstantially cylindrical opening 46 which terminates at its lower endin an annular wedge surface 48 having a central bore 50. Drawingimplement 42 is received within opening 46 with its operative tip 52extending through bore 50.

Transmitter device 40 also includes a retainer 54 in the form of a coverattachable to the upper end of opening 46 to retain drawing implement 42in position within housing 44. Retainer 54 features a spring element 56for biasing drawing implement 42 towards annular wedge surface 48. Anultrasound transmitter 58 is mounted on the lower surface of housing 44proximal to bore 50.

It is a particular feature of preferred embodiments of the transmitterdevice of the present invention that they can accommodate drawingimplements of a range of lengths and widths. To this end, spring element56 adjusts to any variations in length, and biases drawing implement 42towards the lower end of housing 44 to ensure a correct position foruse. This biasing, in conjunction with the shape of annular wedgesurface 48, serves to center the front end of a drawing implement of anysize or shape. In addition, spring element 56 is preferably providedwith a shaped abutment surface 60 having features for centering the backend of a drawing implement. Typically, abutment surface 60 has an axialconical projection as shown for centering drawing implements by engaginga rear axial recess which is common to almost all presentation boardpens. Alternatively, abutment surface 60 may be formed with a conicalrecess or other features for centering the back of a drawing implement.

The combination of annular wedge surface 48 and spring element 56 withabutment surface 60 serves to hold drawing implements of a range oflengths and widths in central alignment within cylindrical opening 46without contacting the sides of housing 44. This arrangement makestransmitter device 40 insensitive to variations in drawing implementwidth. The avoidance of frictional contact with the sides of housing 44is also important for efficient operation of a contact-sensingmicroswitch, as will be described below.

It is a particular feature of certain preferred embodiments of thepresent invention that ultrasound transmitter 58 is formed as asubstantially cylindrical piezoelectric transmitter element attached tothe lower end of housing 44 around central bore 50. This arrangementensures that, when in use, the cylindrical transmitter is coaxial withdrawing implement 42, circumscribing a part of drawing implement 42proximal to operative tip 52. As a result of the symmetry of thisarrangement, TOF measurements of the position of drawing implement 42are completely independent of axial rotation of transmitter device 40.Furthermore, the position of operative tip 52 can be determined veryprecisely by adding the radial dimension of transmitter cylinder 58 tothe value calculated from the TOF.

Transmitter device 40 also typically features at least one element of anelectromagnetic communications link, typically an IR transmitter 60, andpreferably about four such transmitters spaced around the lower end ofhousing 44. This ensures that at least one IR transmitter will becorrectly oriented facing an IR receiver mounted on the presentationboard at any time. It should be noted that a reversed arrangement inwhich an IR link is formed with a board-mounted transmitter and device40 carries a receiver also falls within the scope of the presentinvention. Furthermore, the IR link may be dispensed with entirely ifthree ultrasound receivers are used to calculate each position. However,the arrangement described is preferred for providing higher precisionthan a purely ultrasound-based system whilst avoiding the need forcomplex IR signal processing circuitry in the transmitter device.Additionally, the IR transmitter allows transmission of extrainformation such as pen color and the like.

Ultrasound transmitter 58 and IR transmitters 60 are actuated under thecontrol of electronic circuitry which is preferably battery powered.Both the electronic circuitry and the battery are preferably located ina compartment (not shown) of housing 44.

Transmitter device 40 preferably also features a switch for detectingcontact between operative tip 52 and the surface of a writing board.This switch is associated with the electronic circuitry and is employedto actuate ultrasound transmitter 58 and IR transmitters 60. Preferably,the switch is formed as a microswitch positioned to respond to changesin the force applied by drawing implement 42 against annular wedgesurface 48. FIGS. 6A-6C show a preferred construction for such amicroswitch, generally designated 64, constructed and operativeaccording to the teachings of the present invention.

Microswitch 64 is formed from three functional layers. First, a baselayer 66 provides the two terminals of the microswitch, a singleperipheral contact 68 and a set of common contacts 70, spaced-apartaround the center of base layer 66. On top of base layer 66 lies a layerof conductive resilient foam 72 having cut-out holes 74 oppositecontacts 70. A third rigid conducting layer 76 lies above foam layer 72.Conducting layer 76 has small conductive downward projections 78 alignedwith holes 74. An upper cover 80, integrally formed with annular wedgesurface 48, attaches loosely to base layer 66 to unify the structurewhile allowing sufficient vertical motion for operation of the switch.Each layer has a central bore, together corresponding to bore 50 of FIG.5.

In a non-compressed state, conductive contact is made between peripheralcontact 68 and foam layer 72 and between foam layer 72 and upperconducting layer 76. However, the switch remains open since thethickness of foam layer 72 prevents contact between projections 78 andinner contacts 70. When pressure is applied to compress microswitch 64,foam layer 72 becomes compressed until projections 78 come into contactwith inner contacts 70, thereby closing the switch. In principle,release of the pressure allows the foam layer to return to its initialstate, thereby breaking the circuit. However, in practice, therelaxation response time of the foam material is typically quite slow.For this reason, a spring 82 is mounted between base layer 66 and upperconductive layer 76 such that, when the pressure is released, upperconductive layer 76 is lifted immediately to break the circuit.

It will be clear that, when drawing implement 42 is not in use, springelement 56 urges drawing implement 42 downwards against annular wedgesurface 48 to close microswitch 64. When drawing implement 42 is used todraw on a presentation board, a force is exerted on operative tip 52 ofdrawing implement 42 towards housing 44, causing drawing implement 44 torecoil slightly against spring element 56. This reduces the pressureexerted on annular wedge surface 48 the circuit of microswitch 64 opens.The electronic circuitry of transmitter device 40 is responsive at leastto opening of microswitch 64 to affect a signal transmitted bytransmitter device 40.

FIG. 6B shows microswitch 64 assembled, together with ultrasoundtransmitter 58 and IR transmitters 60. FIG. 6C shows a pair of springelements 84 which are mounted within annular wedge surface 48 so as togrip the end of a drawing implement inserted through central bore 50.This ensures that the upper layer of microswitch 64 is sensitive tomovements of drawing element 42.

It should be noted that the structure described here for microswitch 64is by way of example only. Alternative structures may be used such as,for example, a switch based on a piezoelectric pressure sensor or thelike.

Finally with regard to microswitch 64, it should be noted that correctoperation of the switch depends on a degree of freedom of axial motionof drawing implement 42 against spring element 56. For this reason, itis important that spring element 56 is not fully compressed whenretainer 54 is attached. FIG. 7 shows an example of a preferredstructure for attachment of retainer 54 to housing 44, in which lateralprojections 86 engage channels 88 which are shaped to provide a marginof release 90 when fully engaged. Margin of release 90 is designed to beat least sufficient to allow an operative range of motion of microswitch64.

A second preferred embodiment of transmitter device 40 is shown in FIG.11. Similarly to the preferred embodiment shown in FIG. 5, device 40 isintended for use with drawing implement 42. Transmitter device 40 alsofeatures housing 44 with cylindrical opening 46. However, cylindricalopening 46 now terminates at its lower end with a gasket 134. Gasket 134features a central bore 136, through which operative tip 52 of drawingimplement 42 extends.

In place of retainer 54, transmitter device 40 features a different typeof retainer, which is a holder 138. Holder 138 is hingedly attached tothe upper end of housing 44 with a hinge 137, and acts to hold drawingimplement 42 substantially centered within opening 46. Holder 138 locksonto housing 44 by a locking pin 139. Holder 138 features a springelement 140 for biasing drawing implement 42 towards gasket 134. Asecond spring element 142, located near gasket 134, preferably helpsfurther bias and center drawing implement 42 within opening 46.Preferably, spring element 140 is stronger than second spring element142. A cover 143 is also provided for drawing implement 42.

To retain drawing implement 42 in the centered position, holder 138preferably has an upper biasing element 144. Upper biasing element 144can be in one of two shapes, as shown in FIGS. 12A and 12B. FIG. 12Ashows upper biasing element 144 with an axial conical projection 146 forcentering drawing implement 42 by engaging a rear axial recess 148 whichis common to most presentation board pens. However, this embodiment ispotentially restricted to use only with presentation board pens havingaxial recess 148 with a particular diameter, as axial recess 148 is notof uniform diameter between pens. Alternatively and preferably, upperbiasing element 144 features a recess 150 into which the upper end ofdrawing implement 42 is inserted, as shown in FIG. 12B. This secondembodiment has the advantage of being usable with most presentationboard pens, since the external diameter of these pens is generallyuniform.

The combination of upper biasing element 144, gasket 134 and springelements 140 and 142 has the advantage of holding drawing implements ofa variety of lengths and external diameters in central alignment withincylindrical opening 46 substantially without contacting the sides ofhousing 44. As described above for FIG. 5, the avoidance of frictionalcontact with the sides of housing 44 is also important for efficientoperation of a contact-sensing microswitch 152.

Holder 138 also has a pressure-sensitive element 152, which has twoparts, a pin 154 and a printed circuit board 156. Pin 154 contacts upperbiasing element 144, sensing when contact is made between drawingimplement 42 and the presentation board. In combination, these two partsallow transmitting device 40 to sense when contact has been made withthe presentation board.

Transmitting device 40 also features ultrasound transmitter 58 and IRtransmitter 60, similar to the embodiment shown in FIG. 5. Ultrasoundtransmitter 58 and IR transmitters 60 are actuated under the control ofelectronic circuitry 158 which is preferably battery powered by abattery 160. Both electronic circuitry 158 and battery 160 arepreferably located in holder 138 of housing 44.

Turning to FIGS. 8A-8C, a preferred transmission profile of transmitterdevice 40 will now be described. FIG. 8A represents a contact profile ofdrawing element 42 as measured by microswitch 64 as a function of time.During a first period 100, drawing implement 42 is kept in contact withthe presentation board for an extended period to draw a continuousshape. Then, during a second period 102, drawing implement 42 is used ina series of short, separate strokes to form a dashed line.

As mentioned above, the prior art digitizer systems suffer from asignificant delay in picking-up the beginning of each pen stroke. Thisis because the transmitters are actuated each time pen contact is madeand interrupted each time pen contact ceases. As a result, each penstroke starts with a dead time during which the receiver systemsynchronizes and locks on to the transmitted signals. The results ofthis system are shown in FIG. 8B. During period 100, the effects are notvery serious. There is a small signal loss at the beginning of theperiod, but the great majority of the stroke is recorded well. Duringperiod 102, however, the system response time is comparable to thelength of the pen strokes. As a result, the dashed line is almostcompletely lost.

To solve this problem, the present invention is preferably designed tomaintain synchronization between transmitter device 40 and the receiversystem for a given period after the end of each pen stroke. Typically,this is achieved by the electronic circuitry continuing to operate IRtransmitters 60 for the given time interval after microswitch 64 ceasesto indicate a force exerted on the outer housing towards the operativetip of the drawing implement. False drawing signals can be avoidedeither by the electronic circuitry disabling ultrasound transmitter 58during the delay period, or by changing the content of the IR signal toindicate a non-contact pen state. The delay period is typically at leastabout half a second, and preferably between about 1 and about 2 seconds.

FIG. 8C illustrates the response profile of transmitter device 40 asdescribed. During an initial period of a single pen stroke, its responseis not dissimilar from that of the prior art. However, when shortrepeated strokes are encountered, transmitter device 40 maintainssynchronization between successive strokes, thereby providing anaccurate response immediately on switching of microswitch 64.

Turning now to FIG. 9, an eraser, generally designated 104, constructedand operative according to the teachings of the present invention, foruse with a presentation board digitizer system will be described. Amajor problem with eraser elements for use with digitizer systems is thecommon practice of employing only a part of the eraser surface. Sincethe digitizer is typically unable to distinguish between flat contactand edge contact of the eraser, the digitized image frequently shows amuch greater erased area than has actually been cleared from thepresentation board itself. To solve this problem, eraser 104 isconstructed such that its eraser surface is self-orienting to lieparallel to the presentation board surface. This ensures that thecontact area of the eraser element is always precisely defined.

Thus, eraser 104 has a handle 106 and an eraser element 108 which has asubstantially flat, eraser surface 110. Handle 106 and eraser element108 are connected by a pivot joint 112, typically in the form of aball-and-socket, which has two degrees of rotational freedom. The use ofpivot joint 112 ensures that, in use, eraser element 108 assumes anorientation with eraser surface 110 parallel to the presentation boardsurface substantially independent of the orientation at which handle 106is held.

Eraser 104 also features transmitter device features analogous to thoseof transmitter device 40 described above. These include a cylindricalultrasound transmitter element 114, a number of IR transmitters 116 andan electronic circuitry/battery block 118. Connection of handle 106 topivot joint 112 is through a sprung pin assembly 120. A pressure sensingmicroswitch 122 is mounted in the seat of pin assembly 120 for sensingcontact pressure between handle 106 and eraser element 108. Wiring fromelectronic circuitry 118 to transmitters 114 and 116 is preferablylocated axially within pin assembly 120 and passing through pivot joint112.

Eraser surface 110 is preferably circular, and cylindrical ultrasoundtransmitter element 114 is preferably arranged such that its axis isaligned with the center of eraser surface 110. By addition of the radiusof the cylinder to the TOF measurements, this arrangement allows preciseidentification of the center of the circle of erasure, and hence of theentire area covered by eraser surface 110. Eraser 104 thus provides amuch higher degree of precision and determination of the erased areathan can be achieved by prior art devices.

A second embodiment of an eraser 162 is shown in FIG. 13. Eraser 162 isdesigned for erasing a small area, particularly an area of narrow width,and can thus be described as a “narrow-band eraser.” Similarly to eraser104, eraser 162 has a handle 164 and an eraser element 166 which has asubstantially flat eraser surface 168. However, handle 164 is connectedto eraser element 166 by a pressure-sensitive element 170.Pressure-sensitive element 170 includes a spring 172, such that when atleast a portion of eraser surface 168 contacts the presentation board, asignal is transmitted to a touch switch 174. Touch switch 174 preferablyincludes a printed circuit board 176 and electrical circuitry 178, whichenable touch switch 174 to identify when eraser surface 168 iscontacting the presentation board. This is similar to pressure sensingmicroswitch 122 of eraser 104.

A second method of identification of touching of the presentation boarduses the following features of eraser 162. Eraser surface 168 has twocontact microswitches 180, preferably located substantially at each endof eraser surface 168, which are substantially similar in function tocontact microswitch 64 of FIG. 6. If only one contact microswitch 180senses contact with the presentation board, only a small area will beerased, such as a letter, for example. If, however, both contactmicroswitches 180 sense contact with the presentation board, a zone withthe length and width of eraser surface 168 will be erased.

Similarly to eraser 104, eraser 162 also has transmitter devicefeatures. Specifically, eraser 162 has at least one, and preferably two,cylindrical ultrasound transmitters 182, located in handle 164,preferably substantially at each end of handle 164. Since eachultrasound transmitter 182 is located in handle 164, eraser 162 alsofeatures an ultrasound conductor tube 184 for each ultrasoundtransmitter 182. Each ultrasound conductor tube 184 goes from handle 164to eraser element 166, such that the ultrasound signal from eachultrasound transmitter 182 is transmitted downward. Eraser 162 also hasa reflector cone 186 for each ultrasound transmitter 182. Reflector cone186 is preferably located in eraser element 166, reflecting theultrasound waves in all directions.

Eraser 162 also has two infrared transmitters 188, preferably locatedsubstantially at each end of handle 164. Each infrared transmitter 188has an infrared reflector 190, also located in handle 164, which servesa similar function as reflector cone 186.

Although one particular embodiment of these transmitter device featureshas been described, it will be appreciated that a number of differentembodiments are possible, substantially as described above for thetransmitter device.

Turning now to FIG. 10, this shows the principle of operation of afurther embodiment of a transmitter device, constructed and operativeaccording to the teachings of the present invention, for use with apresentation board digitizer system. This device is generally similar totransmitter device 40 described above except that it dispenses withmicroswitch 64, instead identifying pen-board contact by transmission ofvibrations through the board.

As mentioned earlier, digitizer systems employing through-the-boardtransmission suffer from poor accuracy and dependency on specific boarddesign. However, they have a major advantage of inherent pen-boardcontact identification. The device of the present invention combinesthis feature with all the advantages of precision and independence fromboard design provided by air-borne ultrasound systems, using thethrough-the-board detection solely for contact detection.

Thus, this embodiment may be used with a presentation board systemessentially similar to that of FIG. 1, with the addition of a transducerassociated with the board (not shown) for detecting vibrations from thetransmitter conducted through the board. The processor of the receiversystem is then responsive to outputs from the airborne ultrasoundreceivers to calculate a current position of the transmitter, and to theoutput from the board mounted transducer to identify contact between thedrawing implement and the board, thereby identifying operative strokesof the drawing implement.

The principle of this system is shown clearly in FIG. 10 in which plot130 represents the signal from one of the ultrasound receiver assembliesand plot 132 represents the signal from the board-mounted transducer.Plot 130 shows a continuous sequence of pulses since the transmittersoperate continuously as long as the pen is in use, according to thisembodiment. Plot 132, on the other hand, only registers correspondingpulses during a period that the pen is in contact with the board.Although the signal quality of plot 132 is typically inferior, it ismore than sufficient for identification of contact or non-contactconditions.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe spirit and the scope of the present invention.

What is claimed is:
 1. A system for use with a surface, the systemcomprising: a plurality of receiver assemblies comprising a firstultrasound receiver located adjacent to the surface, and a secondultrasound receiver displaced from the first ultrasound receiver in adirection substantially perpendicular to the surface, the first andsecond ultrasound receivers being connected to generate a total outputsignal corresponding to the instantaneous sum of the ultrasound signalsreceived at each of the first and second ultrasound receivers; anattachable strip, wherein the plurality of receiver assemblies aremounted to the strip; and a movable transmitter device comprising anultrasound transmitter.
 2. The system of claim 1, further comprising: aninfrared receiver mounted on the strip; and wherein the movabletransmitter device further comprises an infrared transmitter.
 3. Thesystem of claim 1, wherein the movable transmitter device furthercomprises: a housing comprising a substantially cylindrical openingterminating at a first end comprising an inner housing surface having acentral bore, the housing receiving a portion of a drawing implementthat comprises a body, a back end, a front end opposite the back end,and an operative tip that extends through the central bore, a retainerattachable to a second end of the opening to retain the drawingimplement within the housing, the retainer having a spring element forbiasing the drawing implement towards the inner housing surface; andwherein the ultrasound transmitter is mounted relative to the housingproximal towards the central bore.
 4. The system of claim 3, wherein theultrasound transmitter is a substantially cylindrical piezoelectrictransmitter attached to the housing around the central bore.
 5. Thesystem of claim 3, wherein the housing further comprises: a microswitchactuated by changes in pressure exerted on the surface so as to beresponsive to a force exerted on the operative tip of the drawingimplement towards the housing; and electronic circuitry responsive tothe microswitch to affect operation of the ultrasonic transmitter atleast when the microswitch indicates a force exerted on the operativetip of the drawing implement towards the housing.
 6. A process for usewith a surface, the process comprising: providing at least threeultrasound receivers associated with the surface; transmitting anultrasound signal from an element movable relative to the surface;receiving the ultrasound signal at the plurality of ultrasoundreceivers; and analyzing outputs from the ultrasound receiverscomprising the step of identifying as a current position a weightedcentroid of at least a first calculated position derived from theoutputs of a first pair of the receivers and a second calculatedposition derived from the outputs of a second pair of receivers, whereinthe weighting varies as a function of approximate position relative tothe ultrasound receivers.
 7. The process of claim 6, further comprisingtransmitting an infrared signal from the element.
 8. The process ofclaim 7, wherein the infrared signal comprises synchronizationinformation.
 9. The process of claim 7, wherein the infrared signalcomprises pen color information.
 10. The process of claim 6, wherein theultrasound receivers are substantially collinear, and wherein theweighting varies linearly with distance in the direction of alignment ofthe ultrasound receivers over at least a given switch-over zone.